The claimed systems and methods relate generally to audio feedback detectors and feedback interrupters, and more particularly to audio amplification systems that include a feedback loop and a detector of non-building, or ringing, feedback, those systems including public address systems and other electronic devices such as hearing aids.
Feedback in public address systems is a common problem. For background and referring to FIG. 1, a public address system includes a microphone 3 for picking up the speech of a participant 5 and a speaker 2 for broadcasting that speech into a room where others may hear. A public address system also includes electronics 1 that may include several functions, one of which is the amplification of sound received at microphone 3 and produced at speaker 2 by way of an amplifier 4. Usually, microphone 3 and speaker 2 are located in the same audible space as the person speaking 5 and his listeners, and some of the sound produced by speaker 2 is picked up by the microphone 3. For most sound, this is not a problem because the speaker-produced sound has much less volume than the speech of a participant 5 at microphone 3 and because speaker 2 is normally not pointed at microphone 3. However, there may be one or more objects 6 that reflect speaker-produced sound to the microphone 3 that may cause difficulty.
In such a circumstance, a particular frequency of sensitivity may result having a period of the feedback loop (or fraction thereof), which is generally the time for sound to pass from speaker 2 to object 6 and back to microphone 3. Such a feedback path may exist at one frequency, or there may be multiple objects, feedback paths, frequencies and harmonics in an environment. Feedback paths may also arise, diminish or change frequency as objects are moved within the room. Feedback, either howling or ringing, often requires some trigger sound to produce audible effects, although it is possible for low-volume noise to initiate feedback in some systems.
Now shown in FIG. 2 is a representation of a waveform of a typical howling event, which waveform typically represents the voltage produced at amplifier 4, the current through speaker 2 or the voltage signal produced by microphone 3. A howling event is usually initiated by an independent sound 11 passing through the system, i.e. independently of the PA system. (For reference, in the examples of FIGS. 2 through 5 independent sound 11 is shown in equal amplitude.) Feedback is more likely if independent sound 11 contains a frequency component at the frequency of sensitivity, although other sounds can also trigger a feedback event. The electronics of a public address system may also provide some suppression for sounds at low amplitude, for example electronic noise, preventing the triggering of a feedback event until a sound is present that exceeds some level. Moreover, many systems introduce a non-linearity into the system that may block feedback where sound of certain amplitudes are introduced, for example through the inclusion of a compressor or other processing element. Perhaps the most common way to reduce feedback is by control of the gain of the system (reducing it below 1), which may be accomplished for all frequencies simply by volume control or in a frequency range by the use of an equalizer.
Continuing with the example of FIG. 2, at some time 12 the independent sound 11 ceases in this example, and the remaining portion of the waveform represents the feedback of the system. In the example of FIG. 2 the gain at the frequency of sensitivity is greater than one, causing a signal at the frequency of sensitivity to build in amplitude in region 13. Without remedial measures this amplitude may build until amplifier 4 reaches saturation or until some other system amplification limit is reached.
Still referring to FIG. 2, the region 14 of saturation is referred to commonly as howling, which is characterized by a loud and possibly uncomfortable monotone produced and maintained by the system until the feedback loop is interrupted. Automatic remedial measures have been devised to detect and interrupt howling, which generally use a methodology as follows. First, the amplitude of sound produced at speaker 2 is monitored, and if the sound exceeds some threshold for a period of time howling is detected. The remedial measure may be to reduce the gain of amplifier 4, or to identify the frequency of howling and apply a filter, such as a notch filter or a filter applying a phase change to cancellation at the frequency of howling. More sophisticated systems apply a discrete Fourier transform to the signal, looking for sound around a particular frequency that exceeds a threshold for some specified period. However, all of these systems must wait for a period of howling to occur, and are not capable of proactively suppressing feedback.
For example, shown in FIG. 3 is a waveform representative of a feedback event where the gain in the feedback loop is over, but close to one. Under those conditions the feedback at the frequency of sensitivity slowly builds in volume, and in a thresholded system no remedial measures will be engaged for some period of time because the amplitude of the feedback signal is close to the amplitude of participant speech. The situation shown in FIG. 3 of a gain of just over one is referred to herein as moderate feedback as opposed to the situation of FIG. 2 of aggressive feedback that reasonably rapidly saturates or howls from the standpoint of a volume-threshold detection method.
Now turning to FIG. 4, other feedback events may not build into howling, but rather feedback may decay in a “ringing” fashion. This type of feedback can be initiated by a frequency component at or near the frequency of sensitivity in the independent speech 11. The feedback effect 15 is sometimes heard as a monotone “ringing” after each word or phrase of speech by the participant. The prior remedial measures for howling are not effective to ringing, generally because the amplitude of the ringing is less than the amplitude of participant speech and is more difficult to detect.
Ringing of the form shown in FIG. 4 in system operation is apparent, extending noticeably some time after independent speech ends. Weak-ringing such as that shown in FIG. 5 may decay more rapidly, but although might not be immediately identifiable by a person may also be a nuisance; muddying speech and fatiguing the ears of listeners. Of note, sometimes ringing can have an oscillatory behavior, that is not decreasing strictly in a logarithmic way; rather some ringing can be influenced by other noise or distorting factors, including electronic noise or acoustic noise in the environment near the frequencies of ringing, automatic gain controls, or dynamic filters. Ringing can also be influenced in the short term by the movement of reflective and absorptive objects in the environment, such as the movement of a person close to a microphone or a speaker. If these ringing-influencing factors are present, systems of feedback detection such as those described herein may be designed to provide more reliable detection by allowing for some deviation from a logarithmic or steady decay, or by allowing for interruptions or variations in the rate of decay of feedback that may be present.
Although not necessarily prior-art, the following references are presented to help to understand and appreciate the systems and methods described herein, each of which references is hereby incorporated by reference as background material. U.S. Pat. No. 6,798,754 to Farhang-Boroujeny, U.S. Pat. No. 5,442,712 to Kawamura et al., U.S. Pat. Publ. No. 2004/0179387, U.S. Pat. No. 5,717,772 to Lane et al., U.S. Pat. No. 5,245,665 to Lewis et al. and U.S. Pat. Publ. No. 2006/0159282 to Borsch.
U.S. Pat. No. 6,798,754 to Farhang-Boroujeny describes a howling detector that uses frequency bins, whereby if the bins manifest a frequency level over a threshold for a period of time howling is detected, and attenuation is applied at the bin-frequency of howling. The frequency bins are charged through the use of frequency decomposition using a Fast-Fourier Transform. Farhang-Boroujeny's method is capable of detecting howling on more than one frequency, as the threshold is applied on a per-bin basis.
U.S. Pat. No. 5,442,712 to Kawamura et al. discloses a howl suppressor and detector also using frequency decomposition in the application of a notch filter at a frequency calculated from detected frequency levels. A digital notch filter may be substituted with its analog equivalent, for example in U.S. Pat. No. 5,245,665 to Lewis et. al. a switched capacitor system is used.
Rather than application of attenuation at the frequency of a detection bin, Borsch uses a simple interpolation method using the levels of adjoining bins to detect more precisely the frequency of feedback.
The application of a FFT is equivalent to the application of a series of band-pass filters. For example, U.S. Pat. No. 5,910,994 to Lane et al. discloses a variation of a frequency detector system that uses a sequence of bandpass filters arranged in a tree.